Steam turbine plant

ABSTRACT

A gas and steam turbine plant includes a steam turbine having a water-steam loop. A waste heat boiler which is connected downstream of a gas turbine includes a high-pressure preheater connected in the water-steam loop, a high-pressure heater connected downstream of the high-pressure preheater, a low-pressure heater connected upstream of the high-pressure preheater, and an evaporator of a medium-pressure heater. A coal gasification plant is connected upstream of the gas turbine. An outflow line leads into the coal gasification plant and is connected to the water-steam loop downstream of the high-pressure preheater, as seen in water flow direction. A water-steam drum communicates with the outflow line and is connected to the evaporator.

This is a division of application Ser. No. 07/916,033, filed Jul. 17,1992 now U.S. Pat. No. 5,251,432.

The invention relates to a method for operating a gas and steam turbineplant, in which a working fluid expanded in a gas turbine is used togenerate steam for a steam turbine, and when coal gas is used as fuelfor the gas turbine, some of the preheated water, which is also at highpressure, is drawn from the water-steam loop of the steam turbine andused for cooling the coal gas. The invention is also directed to a gasand steam turbine plant operating according to the method.

In a gas and steam turbine plant, heat contained in exhaust gas from thegas turbine is utilized in a waste heat boiler to generate steam for thesteam turbine. The water-steam loop of the steam turbine includes atleast two pressure stages, each being made up of a preheater as well asan evaporator and a superheater. In a plant with a coal gasifierconnected upstream of the gas turbine, preheated water is drawn from thewater-steam loop for cooling crude gas generated in the coalgasification. The heat thus recovered is also used to generate steam forthe steam turbine.

In partial-load operation of such a plant, a fresh steam quantity thatis generated decreases disproportionately to the thermal outputcontained in the exhaust gas from the gas turbine, which is availablefor preheating the feedwater. That is particularly true when natural gasis used as fuel for the gas turbine, or in other words in natural gasoperation, because then the feedwater quantity and accordingly the freshsteam quantity as well are reduced by the proportion of water drawn fromthe water-steam loop for coal gasification in coal gas operation. Thedanger exists both in partial-load operation and in natural gasoperation that the disproportionately high amount of available heat inthe waste heat boiler will lead to evaporation, particularly in thehigh-pressure preheater or economizer. Corrosion damage to the heatingsurfaces of the preheater can occur as a result. Moreover, thetemperature of the exhaust gas leaving the waste heat boiler rises toextremely high values, so that the overall efficiency of the plant islimited.

It is accordingly an object of the invention to provide a method foroperating a gas and steam turbine plant and a plant for performing themethod, which overcome the hereinafore-mentioned disadvantages of theheretofore-known methods and devices of this general type and which doso in such a way that regardless of the fuel being used, the highestpossible overall efficiency is attained under all operating conditions,including natural gas operation or partial-load operation, whilethermodynamic losses are avoided.

With the foregoing and other objects in view there is provided, inaccordance with the invention, a method for operating a gas and steamturbine plant having a gas turbine and a steam turbine with awater-steam loop, which comprises generating steam for the steam turbinewith working fluid expanded in the gas turbine; drawing a partialquantity of preheated water at high pressure from the water-steam loopof the steam turbine and cooling coal gas with the partial quantity,when coal gas is used as fuel for the gas turbine; expanding the drawnpartial quantity and superheating steam being formed in the process atmedium pressure, both in partial-load operation and when natural gas isused as fuel for the gas turbine; and evaporating water remaining in theexpansion at medium pressure.

The steam is suitably superheated at medium pressure, along with thesteam formed in the expansion, and delivered in the superheated state toa medium-pressure part of the steam turbine.

Published European Application No. 0 410 111 discloses a gas and steamturbine plant in which the steam for the steam turbine is generated in ahigh-pressure stage and in a low-pressure stage of the water-steam loop.In partial-load operation, the water quantity flowing through theheating surfaces of the high-pressure preheater or economizer isincreased to such an extent that no evaporation can occur there. Apartial quantity of the feedwater, which is at high pressure andpreheated and is not needed for steam generation, is first expanded tomedium pressure in a first expansion stage, and the steam thus formed issuperheated at medium pressure. The water remaining in the expansion isexpanded to low pressure in a second expansion stage, and the steam thusformed is superheated at low pressure. The water remaining in the secondexpansion is returned to a feedwater container integrated into thewater-steam loop. However, thermodynamic losses occur in each expansionstage.

In accordance with another mode of the invention, there is provided amethod which comprises drawing a further partial quantity of thepreheated water, which is at high pressure, from the water-steam loopand evaporating it at the medium pressure. In this process,advantageously all of the water evaporated at medium pressure issuperheated with steam flowing out of a high-pressure part of the steamturbine.

In accordance with a further mode of the invention, there is provided amethod which comprises superheating the water evaporated at mediumpressure along with the steam formed in the expansion, and deliveringthe water and the steam to a medium-pressure part of the steam turbine.

In accordance with an added mode of the invention, there is provided amethod which comprises superheating the steam at medium pressure alongwith steam flowing out of a high-pressure part of the steam turbine.

In accordance with an additional mode of the invention, there isprovided a method which comprises drawing a maximum of 60% of thepre-heated water, at high pressure, from the water-steam loop, in orderto cool the coal gas.

With the objects of the invention in view, there is also provided a gasand steam turbine plant, comprising a steam turbine having a water-steamloop; a gas turbine; a waste heat boiler connected downstream of saidgas turbine, said waste heat boiler including a high-pressure preheaterconnected in said water-steam loop, a high-pressure heater connecteddownstream of said high-pressure preheater, a low-pressure heaterconnected upstream of said high-pressure preheater, and an evaporator ofa medium-pressure heater; a coal gasification plant connected upstreamof said gas turbine; an outflow line leading into said coal gasificationplant and being connected to said water-steam loop downstream of saidhigh-pressure preheater, as seen in water flow direction; and awater-steam drum communicating with said outflow line and beingconnected to said evaporator.

In accordance with another feature of the invention, there is provided avalve connected between said outflow line and said water-steam drum.

In accordance with a further feature of the invention, the steam turbinehas a medium-pressure part, and said waste heat boiler has anintermediate superheater connected between said water-steam drum andsaid medium-pressure part.

In accordance with an added feature of the invention, the high-pressurepreheater has two series-connected heating surfaces.

In accordance with an additional feature of the invention, theevaporator in said waste heat boiler is disposed between said heatingsurfaces of said high-pressure preheater.

In accordance with a concomitant feature of the invention, thewater-steam drum is connected to said water-steam loop between saidheating surfaces of said high-pressure preheater. This is done in orderto adjust the quantity of water in the water-steam drum.

The advantages attained with the invention are in particular that,regardless of the fuel used for the gas turbine, an adequate waterthroughput in all load ranges through the high-pressure preheater oreconomizer is assured, so that evaporation there is reliably avoided.When natural gas is used, as well as in partial-load operation, onlyslight thermodynamic losses occur in only one expansion stage from theexpansion of the partial quantity of water drawn from the economizer.

Other features which are considered as characteristic for the inventionare set forth in the appended claims.

Although the invention is illustrated and described herein as embodiedin a method for operating a gas and steam turbine plant and a plant forperforming the method, it is nevertheless not intended to be limited tothe details shown, since various modifications and structural changesmay be made therein without departing from the spirit of the inventionand within the scope and range of equivalents of the claims.

The construction and method of operation of the invention, however,together with additional objects and advantages thereof will be bestunderstood from the following description of specific embodiments whenread in connection with the accompanying drawing.

The drawing is a schematic circuit diagram of a gas and steam turbineplant, with a water-steam loop of the steam turbine being made up ofthree pressure stages.

Referring now to the figures of the drawing in detail and first,particularly, to FIG. 1 thereof, there is seen a gas and steam turbineplant which includes a gas turbine plant 1a and a steam turbine plant1b. The gas turbine plant 1a is connected downstream of a coalgasification plant 1c. The gas turbine plant 1a includes a gas turbine 2with an air compressor 3 coupled thereto and a generator 4 as well as acombustion chamber 5 being connected upstream of the gas turbine 2 andbeing connected to a fresh air line 6 of the air compressor 3.

The steam turbine plant 1b includes a steam turbine 10 with a generator11 coupled thereto, and a water-steam loop 12 including a condenser 13being connected downstream of the steam turbine 10 and a feedwatercontainer 14 being connected downstream of the condenser 13, as well asa waste heat boiler 15.

The steam turbine 10 includes a high-pressure part 10a, amedium-pressure part 10b and a low-pressure part 10c, which drives thegenerator 11 through a common shaft 17.

In order to supply exhaust gases AG from the gas turbine 2 to the wasteheat boiler 15, an exhaust gas line 9 is connected to one inlet 15a ofthe waste heat boiler 15. The exhaust gas AG leaves the waste heatboiler 15 through an outlet 15b in the direction of a non-illustratedchimney.

The waste heat boiler 15 includes a high-pressure preheater oreconomizer 20, which is made up of two series-connected heating surfaces20a and 20b; a low-pressure heater 22, 23; a medium-pressure heater 24,25; and a high-pressure heater 26, 27.

The low-pressure heater 22, 23 includes an evaporator 22 and asuperheater 23, which together with the low-pressure part 10c of thesteam turbine 10 and with a preheater 28 and a water-steam drum 34 forma low-pressure stage of the water-steam loop 12.

The high-pressure heater 26, 27 includes an evaporator 26 and asuperheater 27, which together with the high-pressure part 10a of thesteam turbine 10 and with the economizer 20 and a water-steam drum 40form a high-pressure stage of the water-steam loop 12.

The medium-pressure heater 24, 25 includes an evaporator 24 and anintermediate superheater 25, which is connected on the input side to thehigh-pressure part 10a through a steam line 42 and on the output side tothe medium-pressure part 10b of the steam turbine 10 through a steamline 44. The evaporator 24 is disposed in the waste heat boiler 15 in aregion between the heating surfaces 20a and 20b of the high-pressurepreheater 20. The medium-pressure heater 24, 25, together with themedium-pressure part 10b of the steam turbine 10 and a water-steam drum54, form a medium-pressure stage of the water-steam loop 12.

An outflow line 46, which leads into the coal gasification plant 1cthrough a valve 48, is connected to the water-steam loop 12 downstreamof the heating surfaces 20b of the economizer 20, as seen in the flowdirection of the water. A compensation line 50 which discharges into thewater-steam drum 54 through a valve 52, is connected to the outflow line46.

When the gas and steam turbine plant is in operation, the combustionchamber 5 is selectively supplied with fuel in the form of coal gas KGfrom the coal gasification plant 1c through a supply line 60, or in theform of natural gas EG through a supply line 62. Upon a transition fromnatural gas operation to coal gas operation, a flap 64 is actuated,which is located in the supply line 60 in the exemplary embodiment.

The fuel KG or EG is combusted in the combustion chamber 5 along withcompressed fresh air L from the air compressor 3. Hot flue gas RGproduced in the combustion serves as working fluid and is carried intothe gas turbine 2 through a flue gas line 8. There it expands, and in sodoing drives the gas turbine 2. The turbine 2 in turn drives the aircompressor 3 and the generator 4. The hot exhaust gas AG leaving the gasturbine 2 is carried through the exhaust gas line 9 into the waste heatboiler 15, where it is used to generate steam for the steam turbine 10.The steam emerging from the low-pressure part 10c of the steam turbine10 is delivered through a steam line 70 to the condenser 13, where itcondenses. Condensate is pumped into the feedwater container 14 by acondensate pump 72. Water is pumped by a circulation pump 74 from thefeedwater container 14 into the preheater 28 and is preheated there. Thewater that is preheated in the preheater 28 flows back into thefeedwater container 14 through a line 76. Preheated water is pumped outof the feedwater container 14 and into the water-steam drum 34 by alow-pressure pump 78. There the water is evaporated with the aid of theevaporator 22. To that end, with the aid of a pump 35, water is pumpedout of the water-steam drum 34 into the evaporator 22 and from thereback into the water steam drum 34. Steam that is separated out in thewater-steam drum 34 is delivered to the superheater 23 and from thereflows, in the superheated state, through a steam line 80, into thelow-pressure part 10 c of the steam turbine 10.

The water from the feedwater container 14 that is supplied to thehigh-pressure stage is first pumped through a high-pressure pump 82 anda valve 84, into the high-pressure preheater or economizer 20 and heatedthere. The water that is heated in the heating surfaces 20a and 20b ofthe economizer 20 flows through a valve 86 into the water-steam drum 40.There the water is evaporated with the aid of the evaporator 26. Waterfrom the water-steam drum 40 is pumped by a pump 41 into the evaporator26 and from there back into the water-steam drum 40. The steam formed inthe water-steam drum 40 flows through a line 90 into the superheater 27and from there it flows, in the superheated state, through a steam line92 into the high-pressure part 10a of the steam turbine 10. The steamwhich is expanded in the high-pressure part 10a flows through the steamline 42 into the intermediate superheater 25 and from there into themedium-pressure part 10b of the steam turbine 10. The steam which isexpanded in the medium-pressure part 10b of the steam turbine 10 flowsthrough a steam line 94, which is connected to the steam line 80, intothe low-pressure part 10c of the steam turbine 10 along with the steamfrom the superheater 23. Some of the steam flowing out of themedium-pressure part 10b of the steam turbine 10 flows into thefeedwater container 14 through a steam line 98, when a valve 96 is open.

In coal gas operation, in other words when coal gas KG is used as fuelfor the steam turbine 2, a partial quantity a of the preheated water,which is adjustable with the valve 48 and is at high pressure, is drawnfrom the water steam loop 12 downstream of the heating surfaces 20b ofthe economizer 20 and delivered to the coal gasification plant throughthe outflow line 46. The partial quantity a is dependent on the gasifiertype being used in the coal gasification plant, among other factors, andamounts to up to 60% of the quantity of water flowing through theeconomizer. In the coal gasification plant 1c, the preheated water isevaporated in a heat exchanger 100 by indirect heat exchange with thecrude gas or coal gas KG produced in the coal gasification process. Thecoal gas KG is cooled down simultaneously with this evaporation. Thesteam formed in the process flows through a steam line 102, out of thecoal gasification plant 1c and into the superheater 27 of thehigh-pressure stage.

In partial-load operation or in natural gas operation, in other wordswhen natural gas EG is used as fuel for the gas turbine 2, the partialquantity a of the preheated water, which is at high pressure, that isnot needed for the steam generation, flows into the water-steam drum 54through the compensation line 50. The preheated water is first expandedto a medium pressure and the water remaining in the process isevaporated at this same medium pressure in the evaporator 24. To thisend, water is pumped by means of a pump 55 out of the water-steam drum54 and into the evaporator 24 and from there back into the water-steamdrum 54. Next, this steam, along with the steam formed in the expansionand the steam flowing out of the high-pressure part 10a of the steamturbine 10 through the steam line 42, is superheated in the intermediatesuperheater 25. The superheated steam is supplied to the medium-pressurepart 10b of the steam turbine 10 through the steam line 44.

In order to regulate the quantity of water in the water-steam drum 54and to adjust the steam flowing through the medium-pressure stage, afurther adjustable partial quantity b of the preheated water is drawnfrom the water-steam loop 12. To this end, a line 104 having a valve 106and communicating with the water steam drum 54, is connected to theeconomizer 20 in the region between its heating surfaces 20a and 20b.

The operating range required for expanding the water, which waspreheated in the economizer 20 and is at high pressure, is extremelylimited, so that the thermodynamic losses are slight. Moreover, becauseof the evaporation of the water collected in the water-steam drum 54 andthe superheating of the thus-formed steam in the medium-pressure heatingsurfaces 24 and 25, the thermal output of the exhaust gases AG from thegas turbine 2 is optimally used for the steam generation in all loadranges, regardless of the fuel being used. Simultaneously, thetemperature of the exhaust gases AG leaving the waste heat boiler 15 islowered to extremely low values, so that the overall efficiency of theplant 1 is especially high.

We claim:
 1. A gas and steam turbine plant, comprising:a steam turbinehaving a water-steam loop defining a water flow direction; a gas turbinedefining a gas flow direction; a waste heat boiler connected downstreamof said gas turbine as seen in the gas flow direction, said waste heatboiler including a high-pressure preheater connected in said water-steamloop, a high-pressure heater connected downstream of said high-pressurepreheater as seen in the water flow direction, a low-pressure heaterconnected upstream of said high-pressure preheater as seen in the waterflow direction, and a medium-pressure heater with an evaporator; a coalgasification plant connected upstream of said gas turbine; an outflowline leading into said coal gasification plant and being connected tosaid water-steam loop downstream of said high-pressure preheater, asseen in water flow direction; and a water-steam drum communicating withsaid outflow line and being connected to said evaporator and saidhigh-pressure preheater.
 2. The plant according to claim 1, including avalve connected between said outflow line and said water-steam drum. 3.The plant according to claim 1, wherein said steam turbine has amedium-pressure part, and said waste heat boiler has an intermediatesuperheater connected between said water-steam drum and saidmedium-pressure part.
 4. The plant according to claim 1, wherein saidhigh-pressure preheater has two series-connected heating surfaces. 5.The plant according to claim 4, wherein said water-steam drum isconnected to said water-steam loop between said heating surfaces of saidhigh-pressure preheater.
 6. The plant according to claim 4, wherein saidevaporator in said waste heat boiler is disposed between said heatingsurfaces of said high-pressure preheater.